In recent years, electroluminescent (hereinafter abbreviated to EL) displays composed of EL devices have been expected as displays of the next generation. EL devices are classified into inorganic EL devices and organic EL devices. Since both inorganic and organic EL devices are self-light-emissive (self-light-luminescent), they have the advantage of providing high visibility. Moreover, EL devices, either inorganic or organic, are fully solid-state devices, so that they also have the advantages of having high resistance to impact and of being easy to handle. For these reasons, research is being carried out to attempt to use EL devices practically as pixels on graphic displays, pixels on TV image displays, surface light sources, and so forth.
An organic EL device is a structure in which a laminate of a light-emitting layer made from a fluorescent organic solid such as anthracene and a hole-injection layer made from a triphenylamine derivative or the like, a laminate of a light-emitting layer and an electron-injection layer made from a perylene derivative or the like, or a laminate of a hole-injection layer, a light-emitting layer, and an electron-injection layer, is sandwiched between a pair of electrodes. Of the two electrodes, the one to be situated on the emission surface side is made of a transparent electrode. Such an organic EL device makes use of emission that takes place at the instance electrons and holes injected into the light-emitting layer recombine with each other. Therefore, the organic EL device comes to be operable at a voltage of as low as e.g., 4.5 V, if the thickness of the light-emitting layer is made smaller. This leads to higher speed of response, and also makes it possible to provide high-luminosity EL devices because the luminosity is proportional to the injection current. Furthermore, by changing the type of the fluorescent organic solid to be used for the light-emitting layer, it is possible to obtain emission in all colors in the visible range of blue, green, yellow, and red. Since organic EL devices have the above-described advantages, especially the advantage of being operable at low voltages, studies are now made to put them in practical use, and some organic EL devices have already been practically used for small-sized displays that can be produced relatively easily, such as displays of mobile phones.
Methods with which organic EL devices can attain color display include: (1) the three-color-films-coating method (three-color-films-deposition method) in which films of three luminescent materials that emit lights of three different colors such as blue, red, and green are applied (deposit); (2) the color conversion method (CCM) in which a blue-light-emitting layer is used in combination with a color changing layer (CCM layer) for conversion of color from blue to green and a color changing layer (CCM layer) for conversion of color from blue to red to develop three colors; and (3) the method in which a white-light-emitting layer is used in combination with color filters of blue, red, green, etc. Of these methods, the three-color-films-coating method (1) is most promising if luminous efficiency is taken into account, and has been practically used in the production of mobile phones, personal digital assistants (PDAs), and so on.
These organic EL devices have had the following problem: they have electrodes made from metallic materials, so that when displays composed of organic EL devices are used in bright circumstances, the display contrast is remarkably lowered due to reflection of extraneous light (outside light). In order to cope with this problem, various measures including (A) attaching a circular polarizer to the man-side surface of an organic EL device, (B) using a color filter, and (C) tinting an organic EL device with an achromatic or nearly achromatic color (carrying out so-called tint treatment) are usually taken.
Theoretically, the above measure (A), attaching a circular polarizer to an organic EL device, makes it possible to prevent reflection of extraneous light completely (though this depends also on the angle of incidence of extraneous light), but the circular polarizer absorbs more than half of the light emitted from the organic EL device as well. Thus, the measure (A) has not always been advantageous from the viewpoint of efficiency. On the other hand, the measure (B), using a color filter, is less effective than the use of a circular polarizer in prevention of reflection of extraneous light. However, the use of a color filter makes it possible to control the hue of the light emitted from the organic EL device. Since there are limitations on improvement in color purity by emission itself of organic EL devices, the hue-control function is considered to be quite effective. Particularly, the combination of the three-color-films-coating method and the use of a color filter is the best from the viewpoint of the balance of luminous efficiency, color purity, and prevention of reflection of extraneous light, and organic EL devices produced by using this combination have actually been commercialized.
Incidentally, the EBU (European Broadcasting Union) Standard (the technical standard created by the European Broadcasting Union consisting of broadcasting stations in Europe and north Africa) is widely used. One of the items of evaluation of the performance of a display is whether or not the display has a color reproduction range that fulfills the EBU Standard. Specifically, the proportion of the area of the triangular color reproduction range of a display to be evaluated, relative to the area of the triangle on the CIE (Commission Internationale de l'Eclairage) chromaticity diagram, specified in the EBU Standard, is used to evaluate the performance of the display. That is to say, the performance of a display is shown by the value [the area of the triangular color reproduction range of the display to be evaluated]/[the area of the triangle specified in the EBU Standard]×100(%). This proportion is so-called “EBU proportion” and is widely accepted as the value showing display performance.
On the other hand, in the evaluation using the EBU proportion, the chromaticity itself of each color of red, green, or blue that a display reproduces is not required to meet the chromaticity specified in the EBU Standard. Therefore, even if a display has an EBU proportion of 100% or more, not all the color tones specified in the EBU Standard can be reproduced by the display.
Patent documents concerning liquid crystal displays, referring to the EBU Standard, include Japanese Laid-Open Patent Publications No. 2003-121838 and No. 2005-309306.
As described above, it becomes possible to make hue adjustment when organic EL devices are combined with color filters. However, in the production of full-color organic EL displays using organic EL devices, enlargement of color reproduction range has come to be strongly demanded like in CRT displays and LCDs, as display contents get complicated and peripheral apparatus become sophisticated. More specifically, there is a demand for organic EL displays that can fully meet the color reproduction range specified in the EBU Standard.